Electrical cable and process

ABSTRACT

An electrical cable which can be made by the tandem extrusion process is provided. The electrical cable comprises (1) a core of an electrical conductor, (2) an aluminum shield surrounding the conductor, at least the outer surface treated with a soluble azo compound cross-linking agent for polyethylene and a complex of fumarato chromic (III) nitrate, (3) a layer of a polyolefin having a density within the range of 0.910 to 0.935 surrounding and adhered to the treated surface of the aluminum shield, and polyolefin being polyethylene or a copolymer of ethylene with at least one Alpha -olefin having three to eight carbon atoms (4) an outer jacket of polyethylene containing carbon black surrounding and adhered to the polyolefin layer.

United States Patent 1191 1111 3,816,643 Pechhold 1 June 11, 1974 [5ELECTRICAL CABLE AND PROCESS 3,681,5[5 8/1972 Mildner 174/107 [75]Inventor: Engelbert Pechhold, Chadds Ford, OTHER PUBLICATIONS Chem.Abstract of Belgium Patent 765137, Complex [73] Assignee: E. I. du Pontde Nemours and Chromium Salt Metal Surface Primer in AQS Solu- Company,Wilmington, Del. [22] Filed: Sept 1973 Primary ExaminerBernard A.Gilheany [21] Appl, No.; 397,719 Assistant Examiner-A. T. Grimley [63] IRelated US. Application Data l 57] ABSTRACT gy rsggggl g of 241240 Anelectrical cable which can be made by the tandem extrusion process isprovided. The electrical cable [52 us. c1 174/107 156/54 174/36 mPriSes(1) a we of an electrical cnductor- (2) 1747] 10 an aluminum shieldsurrounding the conductor, at [51 1 1m. CL I "01b 7/18 least the outersurface treated with a soluble azo com-' [58] Field 52 53 poundcross-linking agent for polyethylene and a com- I74/IO7 110 R plex offumarato chromic (III) nitrate, (3) a layer ofa R 12'() C 6 polyolefinhaving a density within the range of 0.910

to 0.935 surrounding and adhered to the treated sur- [56] ReferencesCited face of the aluminum shield, and polyolefin being polyethylene ora copolymer of ethylene with at least UNITED STATES PATENTS one a-olefinhaving three to eight carbon atoms (4) an Tllll outer jacket ofpolyethylene ontaining arbon black gigs? at surrounding and adhered tothe polyolefin layer. 3:674:9l5 7/1972Pritchard.I:I:............:..::.:.... 174/107 15 Claims, 1 DrawingFigure ELECTRICAL CABLE AND PROCESS CROSS REFERENCE TO RELATEDAPPLICATION This is a continuation-in-part of application Ser. No.241,240, filed Apr. 5, 1972 now abandoned, and assigned to the assigneeof the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to electrical cables.

2. Prior Art The conventional technique for making corrosion resistantelectrical cable employs a laminated shield as described in US. Pat. No.3,206,541, issued to Ludwik Jachimowicz on Sept. 14, 1965, and in US.Pat. No. 3,233,036, issued to Ludwik Jachimowicz on Feb. 1, 1966. Thisshield consists of aluminum or copper foil coated on one or both sideswith a copolymer of ethylene and acrylic acid or acrylic acid esters ofthe type described in US. Pat. No. 2,987,501 issued to James K. Rieke etal. on June 6, 1961, and US. Pat. No. 3,027,346 issued to Frank M. Rugget al. on Mar. 27, 1962. The coating exhibits moderate to good adhesionand fair durability to moisture. An additional cable structure isdescribed in US. Pat. No. 3,586,756 issued to O. G. Garner et al. onJune 22, 1971.

Manufacture of electrical cable of the type above described is describedin US. Pat. No. 3,332,138 issued to O. G. Garner on July 25, 1967.Additional manufacturing methods and cable constructions are describedin US. Pat. No. 3,272,912, issued Sept. 13, 1966 to Ludwik Jachimowiczand US. Pat. No. 3,504,102 is sued Mar. 31, 1970 to F. F. Polizzano.

In electrical cable of the type above described, it would be desirableto use polyethylene because of its high electrical resistivity andresistance to chemicals and moisture.

U.S. Pat. No. 3,674,915, issued July 4, 1972 to J. E. Pritchardrecommends the use of certain ethylene-1- olefin copolymers as thedielectric in electric cables, with lubricants or slip agents betweenthe various layers of the cable. US. Pat. No. 3,681,515 to R. C. Mildnerteaches the use of thick or thin adhesive layers of ethylene-acrylicacid copolymer to adhere a polyethylene sheath to an aluminum shield inelectrical cable. However, it would be desirable to have a greater bondstrength than is available with such copolymers.

Belgian Patent 765,137, granted Oct. 1, 1971 discloses the ability offumarato chromic nitrates to adhere polyethylene and aluminum, but evengreater strengths are desirable in electric cable.

SUMMARY OF THE INVENTION According to the present invention there isprovided an electrical cable comprising, (1) a core of an electricalconductor, (2) an aluminum shield surrounding the conductor, at leastthe outer surface treated with a soluble azo compound cross-linkingagent for polyethylene and a complex of fumarato chromic (I11) nitrate,(3) a layer of a polyolefin having a density within the range of 0.9.1to 0.935 surrounding and adhered to the treated surface of the aluminumshield, said polyolefin being polyethylene or a copolymer of ethylenewith at least one a-olefin having three to eight carbon atoms and (4) anouter jacket of polyethylene containing carbon black surrounding andadhered to the polyolefin layer.

BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing is across-sectional view of a cable constructed in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION In the drawing, there is shown anelectrical cable comprising a plurality of individually insulatedconductors 10 making up a cable core. The core is wrapped longitudinallywith an aluminum foil 11 which is treated on one or both surfaces withan aqueous solution containing an azo acid cross-linking agent forpolyethylene and a complex of fumarato chromic (III) nitrate. Thealuminum foil is folded in such a way that overlap of the edges occursto form lap seam 12. While the aluminum foil can be of any convenientthickness, it will usually be about 5 to 10 mils thick.

The fumarato chromic (I11) nitrate complex used to treat the aluminumsurface is described in US. application Ser. No. 210,833, filed Dec. 22,1971, in the name of Alden J. Deyrup and assigned to the assignee of thepresent application, the disclosure of which is hereby incorporated byreference. A preferred complex is the Type II complex described therein.This complex has the approximate formula:

Cross-linking agents for polyolefms are generally either azo compoundsor peroxide compounds. For commercial applications it is desirable tohave stable treating solutions with long shelf life at a wide range oftemperatures from near freezing to over 40C. such as might beencountered in transporting the materials. Available azo compounds aremore likely than peroxide compounds to be stable in the highly acidic(pH about 1.5 to 2.0) solution of chromium complex.

It is also desirable to have a minimum of organic solvents in thetreating solutions, both to minimize the ecological and economic needfor solvent recovery and also to avoid flammability in the materialsbeing worked with. Treating solutions may desirably contain from about10 to about 30 percent, preferably less than 20 percent, isopropanol andstill not present flammability hazards. Parts and percentages herein areby weight except where indicated otherwise. Thus, the azo compounds mustbe soluble and stable in acidic water containing 10 to 30 percentisopropanol and the chromium complexes. This potentially includes theso-called water soluble azo compounds and some that are not soluble inpure water.

A preferred water soluble compound for use in the invention is4-t-butylazo-4-cyanovaleric acid which has the following structure:

CH; CH:

This material is sold by the Lucidol Company under the designationR-A73.

A less effective but still useful water soluble azo compound is4,4azobis[4-cyanovaleric acid], having the structure:

Another effective water soluble azo compound is the monoamide derivativeof azo bisisobutyronitrile, known as2-cyano-2'-carbamoylazobisisopropane and having the structure:

Other suitable azo compounds will be readily apparent to those skilledin the art. Although various levels of azo compounds are suitable intreating solutions of the invention, 0.1 to 0.4 percent concentrationsare desirable.

Sodium salts and chloride salts are undesirable as the azo compoundsbecause of reaction with the acid chromium complex solutions anddeleterious effects on adhesion.

Application of an aqueous solution of the azo compound and the complexto the aluminum surface can be by any convenient technique such asdipping, spraying, brushing or rolling. After contacting the aluminumsurface with the solution, the surface may be dried by heating. Theeffect of the treatment is to cause an invisible, nonetching surfacealteration, probably the adsorption of a monomolecular layer of thecomplex which improves the speed and durability of adhesion of polymersto the substrate.

The treated aluminum foil which is wrapped around the cable core to formthe shield is then surrounded with an adhesive layer 13 of a polyolefinwhich is usually 1 to mils in thickness but can be 50 mils or more inthickness. The polyolefin used has a density between 0.910 to 0.935.Polyolefins useful in the present invention and which have the specifieddensity are polyethylene and copolymers of ethylene with at least oneoz-olefin having three to eight carbon atoms. The upper limit of theoz-olefin in the copolymer is only determined by the specific a-olefinused which will give a copolymer having a density within the specifieddensity range.

In order to protect the coated aluminum shield, a protective outerjacket 14 of polyethylene filled with carbon black surrounds and adheresto the polyolefin adhesive layer. The outer jacket is usually 40 to 80mils in thickness and is polyethylene having a density between 0.910 to0.935. Carbon black is dispersed throughout the outer jacket at aconcentration lower than the saturation level of carbon black in thepolyethylene, but it will usually be at a concentration between 2 to 3percent.

The electrical cable of the present invention can be manufactured bytechniques known to those skilled in the art. For instance, the methodof British Patent 1,080,778 can be employed by extruding the twopolyethylene layers by the well-known tandem extrusion process.

Advantages of the present invention include the ability to usepolyethylene as the adhesive layer and obtain good adhesion to thealuminum foil. By having the aluminum foil treated with an azo compoundand the complex of fumarato chromic (lll) nitrate, the use of aprelaminated aluminum composite can be avoided. Thus, manufacture of theelectrical cable is simplified be cause of the in-line coating of thealuminum shield with polyethylene by tandem extrusion process.

The invention can be further understood by the following example:

Wettable, dead soft aluminum foil (3 X 0.008 in.) is primed on bothsides with a dilute solution of Type 11 chromium (lll) fumaratecoordination complex as described and prepared in Example 6 of US. Ser.No. 210,833 of Deyrup.

In this example, a solution of Type II is prepared. The preparation isat a total volume of 1 liter, maintained by addition of water to make upfor evaporating losses during boiling.

In a liter of total aqueous solution are placed together 20.0 g.chromium (Ill) nitrate, Cr(NO -9H O, and 40.0 g. of fumaric acid. Thesolution is boiled for 1 hour, then cooled to 24C., resulting in a clearblue solution with pH 1.18. To this are added with stirring 34.0 ml. ofa 1.000 normal sodium bicarbonate solution, resulting in a pH of 1.43.The solution is boiled again 1 hour, then cooled to 24C., resulting in apH of 1.33. To this are added 33.0 ml. of the 1.000 normal sodiumbicarbonate solution, resulting in a pH of 1.63. The solution is boiledagain for 1 hour then cooled to 24C., resulting in a pH of 1.44. To thisare added 33.0 ml. of the 1.000 normal sodium bicarbonate solution,resulting in a pH of 1.84. The solution is boiled 3 hours longer,resulting in a pH of 1.67. It is then cooled to 45C. and filtered toremove excess fumaric acid. The resulting solution had a clear mediumgreen color with an absorption spectrum characteristic of Type 11. Thisexample illustrates the preparation of a fairly completely convertedsolution of 11 which is essentially pure except for the presence ofby-product sodium nitrate.

The Type 11 complex solution is diluted with water containing 10-30percent of isopropanol, such as from 3 to 12 parts per part of solution,preferably about 7: 1. The treating solution of the invention is made byadding azo compound to this complex solution, such as about 0.1 to 0.4percent azo compound, preferably about 0.4 percent based on the weightof the treating solution or 1 to 3 percent based on the chromiumcomplex. Preferably about 0.4 percent of 4-t-butylazo-4- cyanovalericacid is dissolved completely in the solution. This then forms thepreferred treating solution which is ready to use and which has asubstantial shelf life stability. It is applied to aluminum foil in anyof a number of manners, and air-dried in place. A preferred method isthe use of gravure roll application techniques as known in the art.

The treated aluminum foil is used for making cable by a double extrusiontechnique. The foil is fed into a cable wrapping guide where it isfolded with the lap seam extending lengthwise of the cable. No oil isused during the folding process. The resulting aluminum shield is thenpulled into a 2-inch extruder and coated at 230C. at a speed of 15 feetper minute with approximately 0.05 in. of a commercial grade ofunstabilized low density polyethylene having a density of 0.9201 and amelt index of 1.40. The polyethylene is made by coordination catalysisat low pressure with butene-l comonomer. The coated cable is preheated(ca. 50C.), fed into an extruder with a different tubing die, and ajacket of approximately 0.05 inch of black polyethylene having a densityof about 0.93 and containing 2.5 percent carbon black is coated over it.Excellent adhesion is found between the treated aluminum andpolyethylene.

Comparative tests were made with samples of treated aluminum 0.7 milthick adhered to 2.5 mil low density polyethylene film having noantioxidant, Alathon 20 film sold by E. I. du Pont de Nemours andCompany. The tests showed much stronger bonds for samples made with atreating solution of the invention containing azo compound and Type llchromium complex than for samples made with a treating solution whichcontained the Type II chromium complex but not the azo compound.

The tests were made with a 7:1 dilution in water containing 30 percentisopropanol of the above-described Type II chromium complex concentrate.The azo compound was 4-t-butylazo-4-cyanovaleric acid at fourconcentrations. The treating solution was applied to the aluminum foilby means of a gravure roll having 180 quadrangular cells per squareinch, and it was then dried at 65C. The polyethylene was laminated ontothe treated aluminum by passing rapidly over a roll at 204C, with lessthan one second contact time, and then promptly cooled. Then anothersample of each type was produced by passing over the roll at double theinitial speed. Samples were cut from the coated strip thus produced. Thesamples had a width of 1 inch. The aluminum and polyethylene wereseparated if possible and then pulled apart for a 180 separation in anlnstron tester at a jaw separation rate of 2 inches per minute. Certaintests were also made after soaking in water and in a 1:1 by volumewater-ethanol solution for three weeks. The force required to separatethe aluminum and polyethylene is reported in Table l below in grams forthe one inch specimen width.

These data show the superiority of the invention under dry and wetconditions and after exposure to wateralcohol solutions.

What is claimed is:

1. An electrical cable comprising (1) a core of an electrical conductor,(2) an aluminum shield surrounding the conductor, at least the outersurface treated with an aqueous solution containing from about 10 toabout 30 percent by weight isopropanol, and a dissolved azo compoundcross-linking agent for polyethylene, and a complex of fumarato chromic(Ill) nitrate, (3) a layer of a polyolefin having a density within therange of0.9 l 0 to 0.935 surrounding and adhered to the 6 treatedsurface of the aluminum foil, said polyolefin being selected from thegroup consisting of polyethylene and a copolymer of ethylene with atleast one a-olefin having three to eight carbon atoms and( 4) an outerjacket of polyethylene containing carbon black surrounding and adheredto the polyolefin layer.

2. The electrical cable of claim 1 wherein the azo compound is selectedfrom the group consisting of 4-t-butylazo-4-cyanovaleric acid, 2-cyano-2carbamoylazobisisopropane, and -4,4'-azobis[4- cyanovaleric acid].

3. The electrical cable of claim 2 wherein the azo compound is4-t-butylazo-4-cyanovaleric acid.

4. The electrical cable of claim 1 wherein the aluminum strip is about 5to 10 mils in thickness.

5. The electrical cable of claim 1 wherein the polyolefin layer ispolyethylene about 1 to 10 mils in thickness.

6. The electrical cable of claim 1 wherein the outer jacket is 40 tomils in thickness and is polyethylene having a density within the rangeof 0.9 l 0 to 0.935 containing about 2 to 3 percent by weight of carbonblack.

7. An electrical cable comprising (1) a core of an electrical conductor,(2) an aluminum shield 5 to 10 mils in thickness surrounding theconductor, at least the outer surface treated with 4-t-butylazo-4-cyanovaleric acid and a complex of fumarato chromic (lIl) nitrate, (3) alayer of polyethylene about 1 to 10 mils in thickness, having a densitywithin the range of 0.910 to 0.935 surrounding and adhered to thetreated surface of the aluminum shield and (4) an outer jacket 40 to 80mils in thickness of polyethylene having a density within the range of0.910 to 0.935 which contains about 2 to 3 percent by weight of carbonblack.

8. The electrical cable of claim 1 wherein the treated aluminum shieldhas a lap seam extending lengthwise of the cable.

9. The electrical cable of claim 7 wherein the treated aluminum shieldhas a lap seam extending lengthwise of the cable.

10. A process for making an electrical cable of claim 1 in which the azocompound and complex are applied to the aluminum shield by contactingsaid shield with an aqueous solution of said complex and said azocompound.

11. The process of claim 10 in which said azo compound is selected fromthe group consisting of 4-t-butylazo-4-cyanovaleric acid, 2-cyano-2-carbamoylazobisisopropane, and 4,4'-azobis[4- cyanovaleric acid].

12. The process of claim 11 in which said azo compound is4-t-butylazo-4-cyanovaleric acid.

13. A double extrusion process for making electrical cable in which atreated aluminum shield is wrapped around a conductor core with a lapseam, a layer of polyolefin having a density within the range of 0.910to 0.935 is extruded onto said treated aluminum foil, said polyolefinbeing selected from the group consisting of polyethylene and a copolymerof ethylene with at least one a-olefin having three to eight carbonatoms, and then an outer jacket of polyethylene containing carbon blackis extruded onto said polyolefin layer, at least the outer surface ofsaid aluminum shield having been treated by contacting it with asolution in water and about 10 to 30 percent by weight isopropanol of anazo compound cross-linking agent for polyethylene and a complex offumarato chromic (lll) nitrate to provide adhesion between said aluminumshield and said polyolefin layer.

14. A process of claim 13 in which said azo acid is selected from thegroup consisting of 4-t-butylazo-4- cyanovaleric acid,

carbamoylazobisisopropane,

cyanovaleric acid].

- and 4,4 '-azobis 4-

2. The electrical cable of claim 1 wherein the azo compound is selectedfrom the group consisting of 4-t-butylazo-4-cyanovaleric acid,2-cyano-2''-carbamoylazobisisopropane, and 4,4''-azobis(4-cyanovalericacid).
 3. The electrical cable of claim 2 wherein the azo compound is4-t-butylazo-4-cyanovaleric acid.
 4. The electrical cable of claim 1wherein the aluminum strip is about 5 to 10 mils in thickness.
 5. Theelectrical cable of claim 1 wherein the polyolefin layer is polyethyleneabout 1 to 10 mils in thickness.
 6. The electrical cable of claim 1wherein the outer jacket is 40 to 80 mils in thickness and ispolyethylene having a density within the range of 0.910 to 0.935containing about 2 to 3 percent by weight of carbon black.
 7. Anelectrical cable comprising (1) a core of an electrical conductor, (2)an aluminum shield 5 to 10 mils in thickness surrounding the conductor,at least the outer surface treated with 4-t-butylazo-4-cyanovaleric acidand a complex of fumarato chromic (III) nitrate, (3) a layer ofpolyethylene about 1 to 10 mils in thickness, having a density withinthe range of 0.910 to 0.935 surrounding and adhered to the treatedsurface of the aluminum shield and (4) an outer jacket 40 to 80 mils inthickness of polyethylene having a density within the range of 0.910 to0.935 which contAins about 2 to 3 percent by weight of carbon black. 8.The electrical cable of claim 1 wherein the treated aluminum shield hasa lap seam extending lengthwise of the cable.
 9. The electrical cable ofclaim 7 wherein the treated aluminum shield has a lap seam extendinglengthwise of the cable.
 10. A process for making an electrical cable ofclaim 1 in which the azo compound and complex are applied to thealuminum shield by contacting said shield with an aqueous solution ofsaid complex and said azo compound.
 11. The process of claim 10 in whichsaid azo compound is selected from the group consisting of4-t-butylazo-4-cyanovaleric acid, 2-cyano-2''-carbamoylazobisisopropane,and 4,4''-azobis(4-cyanovaleric acid).
 12. The process of claim 11 inwhich said azo compound is 4-t-butylazo-4-cyanovaleric acid.
 13. Adouble extrusion process for making electrical cable in which a treatedaluminum shield is wrapped around a conductor core with a lap seam, alayer of polyolefin having a density within the range of 0.910 to 0.935is extruded onto said treated aluminum foil, said polyolefin beingselected from the group consisting of polyethylene and a copolymer ofethylene with at least one Alpha -olefin having three to eight carbonatoms, and then an outer jacket of polyethylene containing carbon blackis extruded onto said polyolefin layer, at least the outer surface ofsaid aluminum shield having been treated by contacting it with asolution in water and about 10 to 30 percent by weight isopropanol of anazo compound cross-linking agent for polyethylene and a complex offumarato chromic (III) nitrate to provide adhesion between said aluminumshield and said polyolefin layer.
 14. A process of claim 13 in whichsaid azo acid is selected from the group consisting of4-t-butylazo-4-cyanovaleric acid, 2-cyano-2''-carbamoylazobisisopropane,and 4,4''-azobis(4-cyanovaleric acid).
 15. A process of claim 14 inwhich said azo acid is 4-t-butylazo-4-cyanovaleric acid.